Machine Code Generation

AsmJit is a lightweight library for machine code generation written in C++ language. It can generate machine code for X86, X86_64, and AArch64 architectures and supports baseline instructions and all recent extensions. It has a type-safe API that allows C++ compiler to do semantic checks at compile-time even before the assembled code is generated or executed. It also provides an optional register allocator that makes it easy to start generating code without a significant development effort.

A Minimal Example

The example below demonstrates how AsmJit separates concepts used during code generation. Check out AsmJit's documentation for more details and code snippets.

#include <asmjit/asmjit.h>
#include <stdio.h>

using namespace asmjit;

// Signature of the generated function.
typedef int (*Func)(void);

int main(int argc, char* argv[]) {
  // Runtime designed for JIT - it hold relocated functions and controls their lifetime.
  JitRuntime rt;

  // Holds code and relocation information during code generation.
  CodeHolder code;

  // Code holder must be initialized before it can be used. The simples way to initialize
  // it is to use 'Environment' from JIT runtime, which matches the target architecture,
  // operating system, ABI, and other important properties.
  code.init(rt.environment(), rt.cpuFeatures());

  // Emitters can emit code to CodeHolder - let's create 'x86::Assembler', which can emit
  // either 32-bit (x86) or 64-bit (x86_64) code. The following line also attaches the
  // assembler to CodeHolder, which calls 'code.attach(&a)' implicitly.
  x86::Assembler a(&code);

  // Use the x86::Assembler to emit some code to .text section in CodeHolder:
  a.mov(x86::eax, 1);  // Emits 'mov eax, 1' - moves one to 'eax' register.
  a.ret();             // Emits 'ret'        - returns from a function.

  // 'x86::Assembler' is no longer needed from here and can be destroyed or explicitly
  // detached via 'code.detach(&a)' - which detaches an attached emitter from code holder.

  // Now add the generated code to JitRuntime via JitRuntime::add(). This function would
  // copy the code from CodeHolder into memory with executable permission and relocate it.
  Func fn;
  Error err = rt.add(&fn, &code);

  // It's always a good idea to handle errors, especially those returned from the Runtime.
  if (err) {
    printf("AsmJit failed: %s\n", DebugUtils::errorAsString(err));
    return 1;
  }

  // CodeHolder is no longer needed from here and can be safely destroyed. The runtime now
  // holds the relocated function, which we have generated, and controls its lifetime. The
  // function will be freed with the runtime, so it's necessary to keep the runtime around.
  //
  // Use 'code.reset()' to explicitly free CodeHolder's content when necessary.

  // Execute the generated function and print the resulting '1', which it moves to 'eax'.
  int result = fn();
  printf("%d\n", result);

  // All classes use RAII, all resources will be released before `main()` returns, the
  // generated function can be, however, released explicitly if you intend to reuse or
  // keep the runtime alive, which you should in a production-ready code.
  rt.release(fn);

  return 0;
}

Use Cases

AsmJit has been the preferred choice of many projects for years, because of its API, size, and features. It's ideal for implementing high performance and low latency code generators, be it JIT backends of programming languages, recompilers, or specialized compilers that generate code based on data that is only known at runtime. AsmJit has been also successfully used in other domains such as debuggers, reverse engineering tools, and computer graphics.

Sponsors

AsmJit Team would like to thank all companies and individuals that support its development.